1. Field of the Invention
The present invention relates generally to a power steering system, and more particularly to a torque sensor for automobiles.
2. Description of the Prior Art
In general, for large-scale automobiles, vehicles with low air pressure tires, wide tires or the like, the resistance of the ground to the rotational movement of wheels is great. When the resistance of the ground is great, a large amount of manipulating force is required to manipulate a steering wheel, thereby causing the manipulation of the steering wheel to be difficult. In order to overcome such a problem, a power steering apparatus has been employed in an automobile. In such a power steering apparatus, an auxiliary power unit is mounted on a steering apparatus to reduce the force required to manipulate the steering wheel is reduced by the servo action of the auxiliary power unit.
The steering shaft of an automobile, on which a power steering apparatus is mounted, is comprised of input and output shafts. The input shaft is attached to a steering wheel, while the output shaft is attached to a pinion gear engaged with the rack bar of a tie rod. The input and output shafts are connected to each other by a torsion bar. In such a structure, when the steering wheel is rotated, the output shaft is rotated and, finally, directions of wheels connected to the tie rod are changed by the gear action of the pinion gear and the rack bar. However, when the resistance of the ground to the rotation of the wheels are great, the gear action of the pinion gear and the rack bar is not smooth. Accordingly, the output shaft connected to the pinion gear is rotated less than the input shaft connected to the steering wheel, resulting in the twist of the torsion bar. The degree of the twist of the torsion bar is input to an electronic control unit in the form of a signal, and the electronic control unit operates an auxiliary power unit to compensate for the difference between the rotation of the input and output shafts on the basis of the signal. As a result, the manipulation of the steering wheel is improved. In such a case, the twist of the torsion bar is detected by the torque sensor.
FIG. 1 is a cross section showing a conventional torque sensor for automobiles. As illustrated in FIG. 1, the conventional torque sensor for automobiles is comprised of a torsion bar 3 for coaxially connecting the lower end of an input shaft 1 and the upper end of an output shaft 2, three detection rings 4a, 4b and 4c made of magnetic material and arranged on the connected portions of the input and output shafts 1 and 2 to be regularly spaced from each other, and a barrel-shaped sensor housing 5 within which the detection rings 4a, 4b and 4c are fitted.
The output shaft 2 is connected at its lower end to a pinion gear (not shown) engaged with a rack bar formed on a tie rod (not shown), while the upper end of the input shaft 1 is fixedly attached to a steering wheel (not shown). As the torsion bar 3 connecting the input and output shafts 1 and 2 is twisted by the rotation of the steering wheel, the output shaft 2 is rotated.
The detection rings 4a, 4b and 4c include a first detection ring 4a, a second detection ring 4b and a third detection ring 4c. The first and second detection rings 4a and 4b are fitted on the input shaft, and rotated at the substantially same angle as the steering wheel. The third detection ring 4c is fitted on the output shaft 2 to be rotated together with the output shaft 2. Three toothed portions 4d, 4e and 4f are formed on the lower surfaces of the first and second detection rings 4a and 4b and the upper surface of the third detection ring 4c, respectively.
A temperature compensating detection coil assembly 6 and an magneto-resistance detection coil assembly 7 are arranged in the sensor housing 5 to surround the toothed portions 4d, 4e and 4f of the detection rings 4a, 4b and 4c and form an magneto-circuit in conjunction with the detection rings 4a, 4b and 4c. The temperature compensating detection coil assembly 6 is disposed to surround the gap between the first and second detection rings 4a and 4b, and the magneto-resistance detection coil assembly 7 is arranged to surround the gap between the second and third detection rings 4b and 4c. A spacer 8 is interposed between the temperature compensating detection coil assembly 6 and the magneto-resistance detection coil assembly 7. The opposite area of the toothed portions 4e and 4f of the second and third detection rings 4b and 4c is varied by the twisting of the torsion bar 3, the inductance value of the magneto-resistance detection coil assembly 7 is varied by the variation of the opposite area, and the rotational difference of the input and output shafts 1 and 2 is determined by the measurement of the variation of the inductance value.
The detection coil assemblies 6 and 7 are securely held in position in the sensor housing 5 so as to precisely detect torque applied to the torsion bar 3. In order to secure the position of the detection coil assemblies 6 and 7, a ring-shaped stopper screw 9 having a certain thickness is engaged with the sensor housing 5 under the magneto resistance detection coil assembly 7, so the detection coil assemblies 6 and 7 are held in the longitudinal direction of the detector housing 5. External threads are formed on the circumferential surface of the stopper screw 9, and engaged with the lower portion of the inner surface of the sensor housing 5 on which interior threads are formed.
In the process of engaging the stopper screw 9 with the lower portion of the sensor housing 5, when the stopper screw 9 is brought into contact with the lower surface of the magneto-resistance detection coil assembly 7, the position of the magneto-resistance detection coil assembly 7 and the temperature compensating detection coil assembly 6 may be changed by the frictional force between the upper surface of the stopper screw 9 and the lower surface of the magneto-resistance detection coil assembly 7. Additionally, the sensor housing 5 and the detection coil assemblies 6 and 7 are made of different materials, so they have different thermal expansion coefficients, thereby causing them to be expanded to different extents. The variations in the volumes of the sensor housing 5 and the detection coil assemblies 6 and 7 due to the different thermal expansions cause the sensor housing 5 and the detection coil assemblies 6 and 7 to be damaged. In consideration of the above problem, the detection coil assemblies 6 and 7 are positioned in the sensor housing 5 to be spaced apart from the inner surface of the sensor housing 5. Additionally, when the detection coil assemblies 6 and 7 are secured by the stopper screw 9, the stopper screw 9 is slightly spaced apart from the magneto-resistance detection coil assembly 7.
However, if vibration or impact is transmitted to the interior of the torque sensor through slight gaps between the sensor housing 5 and the detection coil assemblies 6 and 7 and between the magneto-resistance detection coil assembly 7 and the stopper screw 9, the positions of the temperature compensating detection coil assembly 6 and the magneto-resistance detection coil assembly 7 are varied, thereby causing the problem that torque applied to the torsion bar 3 is not precisely detected.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a torque sensor for automobiles, in which the torque sensor of a power steering apparatus is improved, so the positions of detection coil assemblies are not only secured effectively but also the thermal deformation of the torsion bar and the shock of impact applied to the torsion bar are handled without any problem.
In order to accomplish the above object, the present invention provides a torque sensor for automobiles, comprising: a torsion bar for coaxially connecting an input shaft attached to a steering wheel and an output shaft attached to a steering mechanism; a plurality of detection rings mounted on the circumferential surfaces of the input and output shafts; at least one detection coil assembly positioned to surround the gaps between the detection rings; a sensor housing provided to accommodate the detection coil assembly; and a position securing element secured to the sensor housing to be brought into tight contact with the side surface of the detection coil assembly through the side of the sensor housing with a friction plate interposed between the detection coil assembly and the position securing element for securing the position of the detection coil assembly.
Preferably, the position securing element is provided to surround portions of the side surface of the detection coil assembly, and is provided with an opening to allow the position securing element to be inserted into the sensor housing.
Preferably, the friction plate is formed to have the same curvature as that of the side surface of the detection coil assembly, and is formed of rubber to absorb vibration applied to the detection coil assembly and accommodate thermal deformation of the detection coil assembly and the sensor housing.
Preferably, the position securing element is comprised of a body portion having the same curvature as that of the side surface of the detection coil assembly to be brought into tight contact with the side surface of the detection coil assembly, an upper extension portion extended from the upper end of the body portion to be brought in tight contact with the upper surface of the detection coil assembly, a lower extension portion extended from the lower end of the body portion to be brought into tight contact with the lower surface of the detection coil assembly, and a plurality of attachment tabs laterally extended from the side of the body portion to fixedly attach the position securing element to the sensor housing.
Preferably, the attachment tabs and the sensor housing are each provided with a plurality of bolt holes to fixedly attach the position securing element.
Preferably, the body portion of the position securing element and the friction plate are each provided with a through hole so as to allow the interior of the sensor housing to communicate with the exterior of the sensor housing.